The present invention relates to a force measuring device, and in particular to an improved device for measuring the force exerted by jaws of a chuck under both static and dynamic rotary conditions.
A new power operated chuck, for example a 12" chuck, can be expected to have a maximum static gripping force on the order of 10,000 lbs. However, if the chuck is poorly maintained, the gripping force could drop to only 5,000 lbs. At the same time, gripping pressures decline as spindle speeds increase due to centrifugal force acting on the chuck jaws, and when the chuck rotates at, for example, 2500 rpm, the gripping force may drop another 2,000 lbs. In consequence, the original static gripping force of 10,000 lbs. may in actuality be reduced to about 3,000 lbs. at operating speeds.
Workpiece damage can result not only from insufficient chuck pressure that might allow the workpiece to rotate within the chuck or fly free while turning, but also by excessive pressure which can deform or crush the workpiece. Therefore, it is desirable to periodically test chuck gripping forces, so that any decline resulting from normal wear and lack of maintenance can be detected, promoting working efficiency and operator safety, and also so that excessive gripping pressures can be avoided.
Devices are available for measuring clamping forces. However, they tend to be special purpose devices designed to work with either 2-jaw or 3-jaw clamps. A hydraulic dial gauge may be used to read load, and when the device is used for rotating applications, a rotary pressure joint and torque arm keep the gauge stationary so that it can be read. In the case of electrical/electromagnetic strain gauge type devices, either a slip-ring rotary joint, a "radio" transmitter or other complex system is required to cope with rotation. Such devices are inherently complex, expensive and often inconvenient to use.